She certainly didn't show it, winning her first set in 21 minutes on Tuesday night on the way to a 6-0, 6-4 win over Alison Van Uytvanck at the Australian Open.
The 18-time major winner has only once lost in the opening round at a Grand Slam tournament, and this is her 58th. One more Grand Slam title and she'll pass Chris Evert and Martina Navratilova on the all-time list of champions.
"If I could get to 19 in Australia that would be amazing," said the top-ranked Williams, who won the last of her five Australian titles in 2010.
Four-time Australian Open champion Novak Djokovic returned to Rod Laver Arena for the first time as a husband and father, shrugging off the effects of a cold to beat No. 116-ranked Aljaz Bedene 6-3, 6-2, 6-4 in his first-round match.
Djokovic won the first of his seven major titles at the 2008 Australian Open, then won here in three consecutive years from 2011 to 2013. His 25-match winning streak at Melbourne Park ended in a quarterfinal loss last year to Stan Wawrinka, who went on to win the title.
Wawrinka began the defence of a major crown for the first time with a 6-1, 6-4, 6-2 win over No. 100-ranked Marsel Ilhan, taking less than 1 1/2 hours to breeze through the first round.
"It's great, bringing me a lot of memories from last year," Wawrinka said of his return. "It was great to come back here feeling happy, happy with my game."
Top-ranked Djokovic had the subsequent match on centre court and appeared to be tired and still recovering from illness after his match, saying he'd had a "rough two weeks health-wise but I'm getting up there."
His mood changed suddenly when a court-side interview turned to reflections of 2014, when he won Wimbledon for the second time, married his long-time partner, Jelena, and the couple had a son, Stefan.
"Well those two events are the two most beautiful events I experienced in my life," he said. "Stefan, he's a blessing we received ... now I have even more motivation."
Two other men widely considered to be in the next generation of major winners advanced in straight sets, with No. 5 Kei Nishikori beating Nicolas Almagro 6-4, 7-6 (1), 6-2 and No. 8 Milos Raonic of Thornhill, Ont., firing 30 aces in a 7-6 (3), 7-6 (3), 6-3 win against qualifier Illya Marchenko.
No. 9 David Ferrer beat Thomaz Bellucci 6-7 (2), 6-2, 6-0, 6-3, No. 12 Feliciano Lopez struggled past American wild-card entry Denis Kudla 3-6, 6-2, 4-6, 6-2, 10-8, while No. 13 Roberto Bautista Agut, No. 18 Gilles Simon -4 and No. 19 John Isner also advanced.
No. 16 Fabio Fognini lost in four sets to Alejando Gonzalez.
Nishikori is playing at his first major since his run to the U.S. Open, when he became the first Asian man to reach a Grand Slam singles final. He's determined to go one better and win a major.
"Yeah for sure that was one of my best results in my career that gave me a lot of confidence," the Japanese star said. "I beat a lot of top 10 players."
Wimbledon champion Petra Kvitova and No. 6 Agnieszka Radwanska needed only 63 minutes for their straight-sets wins over Richel Hogenkamp and Kurumi Nara, and No. 18 Venus Williams beat Maria Teresa Torro 6-2, 6-2. No. 12 Flavia Pennetta, No. 13 Andrea Petkovic and No. 15 Jelena Jankovic were all eliminated, joining the eight seeded players in the women's draw who were beaten on day one.
Two-time champion Victoria Azarenka continued her comeback from a foot injury with a convincing 6-3, 6-2 win over Sloane Stephens — her third in three years at the Australian Open — and will next have to play U.S. Open finalist Caroline Wozniacki, who beat 18-year-old American Taylor Townsend 7-6 (1), 6-2.
No. 11 Dominika Cibulkova, a finalist here last year, opened with a 3-6, 6-3, 6-1 comeback win against Kirsten Flipkens, while 2011 U.S. Open champion Sam Stosur also advanced.